The numbers in brackets below refer to references listed in the Appendix, the teachings of which are hereby incorporated by reference.
The U.S. Department of Energy (DOE) is sponsoring the development of several different types of integrated coal gasification, combined-cycle (IGCC) systems for generating electric power more efficiently than can be accomplished with present power generation systems [1]. An important feature of an IGCC system is the direct utilization of the hot gasifier product as a gas turbine fuel. By not cooling the gas between the gasifier and turbine, the overall thermal efficiency of the system is greater than it would be otherwise. However, the hot gas must be cleaned to remove coal ash and sulfur compounds before it is utilized as a turbine fuel. Several types of hot gas filters are being developed to remove ash. In the IGCC systems under development the hot gas will be contacted with a solid adsorbent which will remove the sulfur compounds. Although various materials can be used for adsorbing sulfur compounds at high temperature, lime is one of the more suitable materials, and it is readily available and low in cost. In some systems under development which employ fluidized bed gasifiers, lime can be supplied to the gasifiers where it reacts directly with sulfur compounds released during coal gasification. In systems which employ other types of gasifiers it is more appropriate to utilize the sorbent in a separate gas contacting device interposed between the gasifier and the gas turbine. Either fixed bed, moving bed, or fluidized bed adsorbers can be used for this purpose. The fixed bed and moving bed adsorbers would utilize sorbent particles which are considerably larger than those used in a fluidized bed adsorber.
Regardless of the gas contacting method, the reaction of lime with sulfur compounds such as hydrogen sulfide in coal gas converts the lime to calcium sulfide. Since calcium sulfide cannot be placed directly in a landfill where it would react slowly with moisture to release toxic hydrogen sulfide gas, the utilization of lime as a sorbent for sulfur compounds requires the application of a suitable process for converting calcium sulfide back to calcium oxide for either reuse or disposal.
Previous investigations have shown that the conversion of calcium sulfide to calcium oxide by oxidation with air or other oxygen-containing mixtures at high temperature is not straight-forward. When a previous attempt was made to oxidize calcium sulfide particles with a gas mixture containing 6 mol % oxygen at a temperature between 650.degree. and 980.degree. C., some of the calcium sulfide was converted to calcium sulfide and the reaction virtually ceased, leaving a large mount of calcium sulfide unreacted [2]. Apparently, calcium sulfate plugged the particle pores because the molar volume of calcium sulfate is 1.9 times that of calcium sulfide. Consequently, the oxidation treatment left individual particles with an unreacted core of calcium sulfide surrounded by an impenetrable shell of calcium sulfate. Other investigations [3,4] showed that the oxidation of calcium sulfide with oxygen-containing mixtures at temperatures in the range of 1000.degree. and 1350.degree. C. produced particles containing both calcium sulfate and calcium oxide. Only by conducting oxidation at 1450.degree. and 1550.degree. C. was it possible to achieve a high conversion of calcium sulfide to calcium oxide in a reasonable time [5]. Unfortunately, such temperatures are not achieved easily, and the lime would probably be dead burned and unreactive so that it could not be recycled.
To circumvent some of these difficulties, Moss [6,7] conceived a process for converting calcium sulfide into calcium oxide in which particles containing a small mount of calcium sulfide are subjected first to oxidation and then to reduction at 1050.degree. to 1090.degree. C. By treating the particles with an oxidizing gas, at least part of the calcium sulfide is converted to calcium sulfate, and then, when the particles are treated with a reducing gas, the calcium sulfate is converted to calcium oxide. This process is designed to regenerate lime employed in a fuel desulfurization process which involved contacting the fuel with hot lime particles in a fluidized bed reactor. The lime is converted to calcium sulfide which is then treated in an adjoining fluidized bed to regenerate the lime. The solids circulate continuously back and forth between the two fluidized beds. One of the most significant features of this process is that the conversion of calcium sulfide in each pass is low. Moss indicated that particles containing no more an 10 mol % calcium sulfide are preferred. Consequently, a large particle recirculation rate between the two fluidized beds is required to convey a given amount of sulfur from the fuel desulfurization bed to the calcium oxide regenerator. For this application it is not necessary for all or even most of the calcium sulfide to be converted to calcium oxide in any given pass through the regenerator.
The Moss process is unsuitable for treating particles with a large concentration of calcium sulfide because only a small fraction of the calcium sulfide would be converted to calcium oxide in passing through the fluidized bed regenerator described by Moss [6,7]. With his system, only an outer layer of calcium sulfide would be oxidized to calcium sulfate and subsequently reduced to calcium oxide which would leave most of the calcium sulfide intact. Therefore, the Moss process is not suitable for treating coal gasifier waste containing a high level of calcium sulfide or for regenerating a lime-based sorbent containing a large concentration of calcium sulfide. Of course, the larger the sorbent particles, the smaller the fraction of calcium sulfide converted and the poorer the performance of the process. Furthermore, the Moss process cannot be used for testing particles which are too large to be fluidized.
Those concerned with these and other problems recognize the need for an improved process for oxidizing calcium sulfide.